Guardrails are traffic barriers placed along roadsides to screen errant vehicles from hazards behind the barrier. A common guardrail in the U.S. is constructed using a standard steel W-beam mounted on spaced wood or steel posts. Because the W-beam functions primarily in tension when redirecting impacting vehicles, a function of the end is to provide necessary anchorage for the beam to develop necessary tensile forces. In addition, since the guardrail end represents a discontinuity in the barrier system, it is subject to being struck "head-on" by vehicles with small departure angles from the roadway. When struck in this manner, the end might spear the vehicle. Some widely used terminal designs "bury" the W-beam at the end to eliminate spearing, but this design may have shortcomings including causing problems relating to vaulting and rollover due to the vehicle riding up the end, and subsequently becoming airborne.
Another type of highway safety device is the crash cushion device. Highway agencies have been using crash cushion devices at high accident locations for a number of years. These devices absorb the energy of head-on impacts with decelerations that are not life-threatening for design conditions. Because the number of guardrail terminals is quite large, and the impact probability low for most, the states do not have the resources to employ crash cushion devices at most guardrail ends because of their expense.
Development of terminal designs is complicated by the need to minimize end-on resistance for the small car impacts while still providing the necessary strength for full-size car impacts either on the end or downstream of the approach end. Efforts have been made to address this problem. For example, U.S. Pat. No. 4,655,434 to Bronstad, which is incorporated herein by reference for all purposes, discloses an energy-absorbing guardrail terminal having beams with uniformly, vertically-aligned spaced openings to absorb kinetic energy of an impacting vehicle. The resistant forces developed by the '434 guardrail terminal are in the form of impulses as shown in FIG. 3 of the '434 Patent.